Using street view, you can often see "Social Security" on a building or sign.

Social Security Office Directions and Notes :

WE ARE LOCATED DOWNTOWN IN THE FEDERAL BUILDING AT 5TH & BELLE STS WE ARE FIVE BLOCKS NORTH OF THE ALTON BELLE CASINO BOAT ON US 67 (PIASA STREET)

Alton Social Security Office Notes

Social Security office hours have been reduced the past few years.
Now almost all Social Security offices have the same hours of 9:00
to noon on Wednesdays, and 9:00 to 4:00 on other weekdays.
Fridays may be a bit less crowded. Try to get an appointment by
phone first. You can do many things like signup for retirement
benefits online at ssa.gov.